1. Field of Invention
This invention relates generally to bi-directional valves having pressure actuated seats, and more particularly, but not by way of limitation, to bi-directional disc valves.
2. Discussion of the Prior Art
Disc valves are generally inexpensively manufactured and, accordingly, are in widespread use throughout the United States and the rest of the world. However, such valves have been subject to inherent limitations generally related to sealing of the valves. In particular, materials commonly used for seats in such valves; for example, polytetrafluoroethylene, are subject to cold flow so that, over a period of time, the seat takes a set which reduces the force with which the seat bears against the disc of a valve due to its own resilience.
In order to avoid leakage arising from the seat of a valve taking a set, it has become conventional to provide some means for urging the seat against the disc independently of the resilience of the material of which the seat is constructed. In particular, a difference in fluid pressure at inlet and outlet passages of the valve is commonly used to force the seat against the disc.
The use of the pressure differential across a valve for forcing the seat against the disc can result in another problem. To the extent that the seat is sufficiently flexible to effect a good seal, the possibility exists that the seat will be blown out of the seat pocket in the valve body by such differential. Thus, unless provision is made to prevent blowout, a disc valve generally cannot be used for high pressure service.
A conventional approach for constructing a disc valve for high pressure service is to narrow the opening of the seat pocket in which the seat is disposed. Such narrowing can be accomplished via flanges as in the sealing mechanism disclosed by Benware, U.S. Pat. No. 3,642,248, issued Feb. 15, 1972, or via converging pocket walls as in the valve disclosed by Maciulaitis et al., U.S. Pat. No. 4,088,299, issued May 9, 1978. Fluid pressure can be transmitted to the outer periphery of the seat via grooves in the sides of the seat (see Benware) or by shaping the seat to prevent seepage about the sides (see Maciulaitis). Wucik, U.S. Pat. No. 3,986,699, issued Oct. 19, 1976, discloses flanges at the opening of the seat pocket and a seat shaped to permit seepage about the sides of the seat. An O-ring overlaying the seat can be used, as in the above patents, to seal one side of the seat against transmission of fluid pressure on one side of the disc to the outer periphery of the seat.
When a valve is used in an application wherein a wide range of pressure differentials can exist between inlet and outlet passages of the valve, a problem can occur when the valve is designed for high pressure service. The construction of the valve to prevent blowout of the seat when the pressure differential between the inlet and outlet passages of the valve is relatively high can sacrifice flexibility of the seat necessary for good sealing of the valve when a lower pressure differential exists between the inlet and outlet passages.